New Zealand Beekeeper, April 2015
Peter Molan, formerly Professor in Biological Sciences and Director of the Honey Research Unit at the University of Waikato (now retired)
There has been up until now a major misunderstanding of the relationship between different levels of antibacterial activity in manuka honey. It has always been assumed that, for example, a honey with a non-peroxide antibacterial activity (NPA) of 20 is twice as potent as one with a rating of NPA 10, but that is in fact not correct. The fallacy is due to it not being taken into account that the NPA rating starts at 8, not zero, a rating of 8 being the minimum level of activity that can be detected in the assay. This is the same situation as temperature measured on the Fahrenheit scale. A temperature of 100°F (38°C) is not twice as warm as 50°F (10°C), because the Fahrenheit scale starts at 32°F, not zero (see Figure 1), whereas a temperature of 100°C is exactly twice as warm as 50°C because the Centigrade scale starts at zero...
It is my opinion, formed from consideration of all the points made here, that it would be best by far for the rating of activity in manuka honey to be done by the whole industry as originally stated by MPI in their guidelines, which was that only the content of methylglyoxal be shown. This would then simply require education of consumers to have them realise that the antibacterial potency is directly proportional to the level of methylglyoxal. Although in New Zealand and Australia there may be restrictions on marketers making reference to antibacterial activity, it could be done by non-commercial educators like myself. In other countries the ANZFS Code does not apply, so there would be no restriction on such educating. Rating the content of methylglyoxal would overcome the problem of marketers using misleading rating numbers that are not actually for NPA. (The MPI guidelines will now allow numbers to mean anything the marketer defines them as meaning, which could be nothing to do with NPA.) It would also curb the freedom of marketers to mislead consumers by giving rating numbers that are actually higher than the true equivalent to NPA ratings. Additionally it would allow consumers to see the actual value of honey on sale rated “MGO 80” when they see it up against manuka honey on sale with methylglyoxal ratings of 800 to 1,200. Furthermore, rating the methylglyoxal content of manuka honey will let consumers see that honey rated as NPA 5 (83 mg/kg methylglyoxal) has only one tenth of the activity of honey rated NPA 20 (830 mg/kg methylglyoxal).